1. Field of the Invention
The present invention relates to a method of driving an ink jet head and an ink jet apparatus having the above ink jet head, and more particularly to the method which is featured by discharging ink as a droplet from a discharge opening communicated with an ink passage to carry out the recording by pressure generated in the ink by means of an electric-mechanical converting element provided along an ink path, and the ink jet apparatus provided with the above ink jet head.
2. Related Background Art
A so-called Gould type ink jet head as shown in the FIG. 1 has been known as one of the ink jet heads of this kind.
In FIG. 1, numeral 1 designates a nozzle body having a hollow cylindrical configuration whose tip end is squeezed like a conical configuration, numeral 2 designates a filter disposed on an opened portion at one end of the nozzle body 1, and numeral 3 designates a discharge opening formed by opening a conical head portion of the other end of the nozzle body 1. Numeral 4 designates a cylindrical piezoelectric element to be engaged with a portion around the nozzle body 1, the piezo-electric element 4 is adhered and and fixed to the nozzle body 1 by an adhesive material 5 such as epoxy resin or other adhesive. Numeral 6 designates a driving portion capable of generating a pulse of wave configuration which will be explained in detail hereinafter.
By applying a piezo-electric pulse as shown in the FIG. 2 to the piezo-electric element 4 by the driving portion 6, the piezo-electric element 4 is subject to mechanical displacement, and generating the pressure wave around a point P in FIG. 1 of the nozzle 1. This pressure wave transmits into two directions (forward and rearward), along an ink path of the nozzle body 1. When one side of the pressure wave reaches to the discharge opening 3, the ink will be discharged as a droplet from the discharge opening 3 by operation of the pressure wave. At this time, the discharge opening operates as a fixed end for the pressure wave because an inner diameter of the nozzle tube is extremely reduced as this portion, and accordingly the pressure wave is reflected in the same phase. The filter 2 operates as a free end because it has opened portion like a lotus or honeycomb whose opened rate is more than 80%, and accordingly the pressure wave is reflected with a reversal phase.
FIG. 3 shows the state in which the above two pressure waves are transmitted in the nozzle from reflection. A solid line shows a compressed wave, while a broken line shows an expanded wave. As apparent from FIG. 3, the reflection of the compressed wave and expanded wave become respectively a compressed wave and expanded wave, because both of them are reflected in the same phase at the discharge opening 3 and are again reversed at the filter 2 in turn to become respectively an expanded wave and compressed wave.
Due to the relation between the above reflected pressure waves and an ink viscosity as well as a surface tension, and the relation between the above reflected pressure waves and a refill of the ink after being discharged through the filter 2, a meniscus M of the ink I at the discharge opening 3 presents damped vibration with complex behavior as shown in FIG. 4, and then return to its initial state. Then the next driving voltage pulse is biased to the piezo-electric element 4 to discharge the ink droplet succesively to thereby carry out recording of an image or letter or other record.
By changing a voltage of the driving voltage pulse biased to the piezo-electric element 4, ink droplet quantity to be discharged can be adjusted. Density of the picture element is varied in this way and gradient expression of the image is effected. Furthermore, as shown in the FIG. 2, a fall or drop of the voltage pulse is made slow to prevent occurrence of sudden mechanical displacement, so that any large negative pressure would not be generated at the point P.
However, the reflected pressure wave causes various bad influences. In detail, the pressure wave generated by the fall of the voltage pulse is reflected in the nozzle as shown in FIG. 3, and being transmitted with reversing the discharge opening 3 before the time when a tail of the ink droplet discharged from the discharge opening 3 to fly is not separated from the meniscus M, so there is a danger air bubbles B being trapped in the nozzle 1 as shown in and FIG. 5.
At the time t.sub.2 as shown in the FIG. 3, there is a danger that the pressure wave acts to the discharge opening 3 of the nozzle to discharge an unnecessary ink droplets, which deteriorate the quality of the image. For overcoming the above disadvantage, the proposal to cancel the pressure wave of the raised voltage pulse by falling wave configuration at the time t.sub.2 as shown in the FIG. 6 has been tried, for example, in the Japanese Patent Laid-Open Application No. 61-266255.
However, the problem of trapping of the air bubble cannot be avoided sufficiently in the above manner because there is no measure provided to the reflected wave at the time t.sub.1. Meanwhile, in an ink jet recording printer of analogue modulation in which the gradient expression is effected by varying the discharge ink quantity, variable range of the discharge ink quantity is important. Furthermore, to realize an ink jet recording printer capable of effecting the high quality recording, gradient character as well as resolution is important, so it is necessary to discharge the fine or minute ink droplets to record fine dots. On the contrary, in the wave configuration shown in FIG. 6, the pulse width is too large to record the fine dots, and resultantly is not suitable for the ink jet printer of analogue modulation.
Fine dots can be recorded by the pulse wave configuration in which pulse width is properly shortened and the rise is made slow. For reducing the influence of the reflected pressure wave, an idea to bias the sub-pulse later than the main pulse by the time difference t.sub.1 as shown in FIG. 7 to thereby cancel the reflected pressure waves has been proposed. However, since the size of the waves transmitted from two directions to overlap at the time t.sub.1 are different, it is impossible to cancel the reflected pressure waves well by the single sub-pulse, which still leaves bad influence leading to delays in refilling.